1. Waves and Sound
Chapter 16

2. Wave
A wave is a traveling disturbance.
A wave carries energy from place to place.

3. Longitudinal wave wave direction
particle motion
wave direction
Particles move back and forth parallel to the direction the wave is moving.

4. Transverse wave wave direction
particle motion
Particles move back and forth perpendicular to the direction the wave is moving.

5. Periodic waves
A periodic wave consist of cycles or patterns that are produced over and over again by the source.
When the end of the slinky is moved in simple harmonic motion, every segment of the slinky will vibrate in simple harmonic motion.
Transverse wave
Longitudinal wave

6. Periodic wave vocabulary
Wave motion v
Displacement of particles all along the wave
One particle's motion vs. time  T
In the drawings, one cycle is shaded in color.
Amplitude A is the maximum motion of a particle of the wave from
the particle's equilibrium position.
Wavelength  is the length of one cycle of the wave.
Period T is the time required for one complete cycle.
Frequency f is the number of cycles each second.
Wave speed v is the speed that the wave travels.

7. Transverse wave motion along a string
Particles that interact with more force move faster.
Particles with more inertia move slower.
tension
linear density
wave speed

8. Example 2 - Waves on a guitar string
A m long, g, guitar string is pulled tight with a tension force of 226 N.
Find the wave speed.
That is about 1850 miles/hour. Fast!!

9. Sound waves are longitudinal waves

10. Wavelength of a sound wave
Wavelength is the distance between adjacent high density (compression) regions.
Compression regions

11. Sound waves are longitudinal waves
wave direction
source of the sound
Individual air molecules are not carried along with the wave. Particles vibrate back and forth parallel to the wave direction.

12. Frequency of a sound wave
Frequency is the number of cycles in one second.
Your brain interprets the frequency in terms of the subjective quality called pitch.
A pure tone is a sound with a single frequency.

13. Wave speed
Sound waves travel in
Solids
Liquids
Gases
at different speeds.

14. Example 5 - "Rule of Thumb" Thunder & Lightning
A rule of thumb is a useful idea that is approximately true.
Count the seconds between when you see the lightning and when you hear the thunder.
The lightning was about a mile away for every 5 seconds you counted.

15. Speed of sound
GASES
LIQUIDS
SOLIDS

16. Sound waves carry energy
Power of the wave is the amount of energy transported by the wave in one second.
Intensity is the power through one square meter.

17. Example 6 - Sound intensity
120 μW of sound pass through surfaces 1 and 2 that have areas of 4 m2 and 12 m2.
Find the sound intensity at each surface.

18. Intensity for uniformly distributed sound waves
For sound waves that are uniformly distributed in all directions, energy is spread uniformly over a spherical area.
Last slide Tuesday, Nov. 25, 2008

19. Example 7 - Fireworks
A person 640 m away from a fireworks rocket explosion measured a sound intensity of 0.1 W/m2.
Find the power released by the rocket explosion.

20. Sound intensity level
Sound intensity level β compares the intensities of two sounds, one sound intensity usually is the threshold of hearing.
Threshold of hearing
sound intensity level
The smallest change in sound intensity level that people can usually detect is 1 dB.

21. Ear responds differently to different frequencies
Our hearing is most sensitive to frequencies around 3500 Hz.

22. Sound intensity level
Every 10 dB increase in sound intensity level doubles the loudness of the sound.
A sound with 10 times more power is 10 dB higher in sound intensity level, but would only sound twice as loud.

23. About 2½ times louder than with just one person talking.
Example - Sound intensity level
One person talking at a normal conversation volume has an intensity level of 65 dB. What would be the intensity level at your next party when there are 30 people each talking in a similar manner?
Call the sound intensity of the one person talking I1.
With 30 people talking in a similar manner, the sound intensity would be 30 times greater so I30 = 30 I1.
increase
About 2½ times louder than with just one person talking.

24. Sound intensity level comparing intensities Io and I
comparing intensities I1 and I2
damage to your hearing

25. Sound intensity level

26. Hearing damage from high sound intensity levels

27. Loud sounds can quickly damage your hearing.
Sound intensity level
Loud sounds can quickly damage your hearing.
Last slide Tues Nov 24, 2009
Will a 2 hour 120 dB rock concert damage your hearing?

28. Doppler effect source at rest source moving
Doppler effect is the change in frequency and wavelength due to the relative motion of the source and the observer with respect to the sound wave in the air

29. Doppler effect - moving source
The source partially catches up with the previous wave and shortens the length of the wave by Δλ, the distance that the source moves during one wave period T.
new shortened wavelength
distance the source moves
original wavelength 1 2 3
Similarly, when the source is moving away from the observer, the wave is stretched out by the distance Δλ that the source moves during one wave period T.

30. Doppler effect - moving observer
Δx=vot
Doppler effect - moving observer
When the observer was at rest, he experienced fwave cycles each second.
When the observer is moving toward the wave, he moves a distance Δx=(vobserver )(1s) each second. For each wavelength distance moved, he picks up one additional cycle. So the number of extra cycles detected is equal to Δx/λ and the number of extra cycles detected per second is equal to vobserver / .
moving toward the wave
moving away from the wave

31. The End